HATS In Situ Monitoring Program

OVERVIEW

Mixing ratios of nitrous oxide and chlorofluorocarbons have been increasing in the earth's atmosphere as the result of human activity. These compounds are in part responsible for the observed decrease in stratospheric ozone around the globe. The Antarctic ozone hole formed in the austral spring is a manifestation of this interaction. These gases, in particular nitrous oxide, can also affect the temperature of the atmosphere and earth's surface in a manner similar to that of carbon dioxide and methane through the "greenhouse effect". Under NOAA's Radiatively Important Trace Species (RITS) program and Climate and Global Change Program, CMDL is measuring these gases in the atmosphere using in situ gas chromatographs at all four CMDL baseline observatories and at Niwot Ridge, Colorado in collaboration with the University of Colorado.

At the current time measurements of nitrous oxide (N₂O), the chlorofluorocarbons: CFC-12 (CCl₂F₂), CFC-11 (CCl₃F), and CFC-113 (CCl₂F-CClF₂) and the chlorinated solvents: methyl chloroform (CH₃CCl₃) and carbon tetrachloride (CCl₄) are being made once an hour. Surface based mixing ratios of these chemicals are representative of the troposphere because their atmospheric lifetimes are long and the sites are distant from sources. The growth rates of all CFCs and chlorinated solvents have diminished in recent years because of their mandated phase out by the Montreal Protocol and subsequent amendments. In particular CFC-11, CFC-113, methyl chloroform, and carbon tetrachloride mixing ratios have peaked in the troposphere and are now diminishing. CFC-12 growth continues to slow down but has not reached zero yet. This chemical's prior major use in domestic, commercial, and industrial refrigeration and air conditioning precludes a quick end to atmospheric release.

With the banned use of chlorofluorocarbons and chlorinated solvents by the end of 1995, substitute hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) have proliferated. These chemicals are difficult to measure because atmospheric mixing ratios are extremely low (parts per quadrillion and low parts per trillion) and their electron capture potential is considerably reduced as the number of halogen atoms is reduced. To measure the most common of these and other species of interest, a new generation of gas chromatograph has been developed based on the compact, lightweight, low power usage, automated systems designed for trace gas analysis aboard aircraft. These "CATS" (Chromatograph for Atmospheric Trace Species) systems are 4 channel and have the capability of measuring the previous chemicals of interest as well as sulfur hexafluoride (SF₆), HCFC-22 (CHClF₂), perchloroethylene (CCl₂=CCl₂), methyl chloride (CH₃Cl), and methyl bromide (CH₃Br).

CATS systems have initially been installed at existing continental tower sites where sampling lines can be attached to the tower at different heights. Measurements then show boundary layer effects and vertical gradients since the sites aren't always far removed from local sources.

PERSONNEL

Thayne Thompson is the RITS/CATS project leader. Goeff Dutton is the CATS project manager acting as the station liaison, managing equipment deployment and maintenance, and doing the initial data processing and quality control. David Nance is the RITS data manager.

INSTRUMENT HISTORY

• Dec 1983

  1 channel manual gas chromatograph at South Pole, Antarctica

  (N₂O, CFC-12, CFC-11)

• Jun 1986

  2 channel RITS automated gas chromatograph at American Samoa

  (N₂O, CFC-12, CFC-11, CFC-113, methyl chloroform, carbon tetrachloride)

• Oct 1986

  2 channel RITS automated gas chromatograph at Barrow, Alaska

  (N₂O, CFC-12, CFC-11, CFC-113, methyl chloroform, carbon tetrachloride)

• Jun 1987

  3 channel RITS automated gas chromatograph at Mauna Loa, Hawaii

  (same as two channel except a higher precision N₂O channel is added)

  additional channel added to American Samoa

  (high precision N₂O)

• Aug 1987

  additional channel added to Barrow, Alaska

  (high precision N₂O)

• Feb 1988

  2 channel RITS automated gas chromatograph at South Pole, Antarctica

  (N₂O, CFC-12, CFC-11, CFC-113, methyl chloroform, carbon tetrachloride)

• Oct 1994

  4 channel automated CATS gas chromatograph at WITN Tower, Grifton, North Carolina

• Jun 1995

  2 channel automated CATS gas chromatograph at WLEF Tower, Parkfalls, Wisconsin

• Jul 1995

  4 channel automated CATS gas chromatograph at Harvard Forest, Massachusetts

• Aug 1995

  1 channel automated CATS gas chromatograph at Alert, Canada

  (N₂O)

• Jan 1998

  4 channel automated CATS gas chromatograph at South Pole, Antarctica

• Jun 1998

  4 channel automated CATS gas chromatograph at Barrow, Alaska

• Sep 1998

  4 channel automated CATS gas chromatograph at Mauna Loa, Hawaii

• Dec 1998

  4 channel automated CATS gas chromatograph at American Samoa

• Feb 1999

  RITS gas chromatograph at Barrow, Alaska retired

FUTURE

• mid 1999

  A 4 channel automated CATS gas chromatograph system will be deployed to Niwot Ridge, Colorado to operate for a six month overlap period with the aging 3 channel system. After the test period is over the old 3 channel systems will be retired.

ANALYTICAL TECHNIQUE

Air is drawn through a sampling line using a clean pump. This air flushes the gas sampling valve for about 5 minutes. The flushing is then stopped and the sample bleeds down to ambient pressure. The sample is inserted into a stream of inert carrier gas (usually nitrogen or a mixture of argon/methane) which pushes the air through the separation process.

The separation of the sample air is accomplished using gas chromatography. Tiny porous beads, sometimes coated with a liquid, interact with the mixture of molecules impeding there movement either because of their size or their solubility. In our case, this separation column is divided into two parts. When the chemicals of interest move onto the second column, the first column's flow is reversed to clean heavier compounds off before the next injection. This is called "backflushing".

Detection of the halocompounds is by an electron capture detector. A radioactive foil of nickel-63 is inside the pin-in-cup detector. The beta decay (an electron) ionizes the carrier gas forming an electron cloud. Periodically a large positive pulse is applied to the center electrode. This causes the free electrons to move to the electrode where they are measured as a tiny current. When a molecule containing halogen atoms, which has the property of enhanced affinity for electrons, enters the detector, it readily attracts and holds a free electron. The background current is thus reduced. In due time the electron capture molecules are flushed out of the detector and the current returns to its previous level.

The measure of the dip in the current curve is a measure of the amount of chemical present. By periodically injecting gas mixtures containing known quantities of the chemical of interest, calibration of the detector is accomplished.

Recording of the inverted current produces a curve called a "chromatogram".

Halocarbons & other Atomspheric Trace Species 325 Broadway R/E/CG1 Boulder, CO 80303